Our previous studies identified a region on chromosome 4q that was linked to measures of insulin action. A candidate gene in this region is FABP2 which encodes the human intestinal fatty acid binding protein (IFABP). We identified a polymorphism in this gene which results in an alanine (Ala54) to threonine (Thr54) substitution at amino acid 54 of IFABP. We found a significant association between the Thr54-encoding IFABP genotype (frequency= 0.29) and increased fasting lipid oxidation rates and insulin resistance, and have further shown that recombinant Thr54 protein has a higher affinity for long-chain fatty acids as compared to recombinant Ala54 protein. To investigate physiologic consequences of the IFABP substitution, we analyzed fatty acid transport across cells expressing either Ala54 or Thr54 IFABP. Caco-2 cells provided a model system for expression of Thr54 and Ala54 IFABP since these cells functionally mimick polarized small intestinal epithelial cells, but do not endogenously express IFABP. We produced permanently transfected Caco-2 cell lines expressing equivalent amounts of Ala54 and Thr54 IFABP and compared their ability to transport long-chain fatty acids from their apical to basolateral surfaces. Transfected cell monolayers, grown on filter supports, were apically exposed to 3H-long-chain fatty acids for 5 min-48 hours and then the basolateral media was analyzed for 3H lipid that had been transported across the monolayer. At all time points, more 3H lipid was transported across the Thr54-expressing cells as compared to the Ala54-expressing cells. The observed difference in transport of long- chain fatty acids across Thr54-expressing cells as compared to Ala54- expressing cells appears to be specific for molecules that interact with IFABP, since no difference in 14C-glucose transport was observed across these cells. The nature of the basolaterally secreted 3H-lipids components was also analyzed and we found that Thr54-expressing cells secreted a 4-6 fold greater amount of triglyceride to the basolateral media than Ala54 cells. A comparison of the protein structures of Thr54 and Ala54 IFABP is being solved by 3-D NMR in collaboration with J. Sacchettini and J. Hamilton.